Tuning a stringed musical instrument includes the act of making minute adjustments to the tensions of the strings to properly align the intervals between their tones so that the instrument is in tune. Many factors can cause these instruments to go out of tune over time, particularly atmospheric changes. For instance, changes in humidity can affect the pitch of an instrument. An out-of-tune instrument can often be identified by the characteristic beating sound it produces. This fluctuation in the sound intensity may be a result of two or more tones, containing similar frequencies, played together. Fine tuning includes an assessment of the vibration interaction among notes, which may be different for every instrument, and thus in practice involves matching the notes to slightly different pitches from any theoretical standard.